crumble and assembly

Using your fingers, work butter into flour mixture until mixture holds together when squeezed. Break up into small clumps and scatter onto a rimmed baking sheet. Bake, tossing halfway through, until crumble is golden brown, 15–20 minutes. Let cool.

Beat cream in a small bowl to soft peaks and whisk in powdered sugar.

Scoop vanilla and lemon ice cream into glasses and top with roasted rhubarb, whipped cream, and crumble.

DO AHEAD: Crumble can be made 2 days ahead. Store airtight at room temperature.